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Pure Water Engineering Calculators 

These calculators will assist in the interpretation of your pure water data, or in the design of your water treatment equipment.*.

Lime Softening Table... predicts the effect of adding specific softening chemicals on water quality.

RO ProbingAid™... assists in the evaluation of individual reverse osmosis membrane vessel conductivity probing by taking into account the changing salt concentrations in the flowing streams in evaluating probing data.

Water ConductivityCalculator™... estimates the water conductance as based on the entered pH and ion concentrations.

OrificeMaker™ Pressure Drop Calculator... estimates the pressure loss that would result from an entered orifice size and quantity.

Deionized Water Tank CO2 Absorbance Calculator... estimates the equilibrium concentrations of alkaline ions and absorbed carbon dioxide in stored deionized water in direct exposure to the atmosphere.

SARcalculator™...calculates the Sodium Adsorption Ratio (SAR) when the calcium, magnesium, and sodium concentrations are entered.  Increasing SAR values will result in increasing soil hardening over time that will negatively impact plant growth.

SDInormalizer™...normalizes silt density index (SDI) results for variation in water temperature, test pressure, and for excessively long test periods.

*Because of the possibility for calculation errors, purewatercloud cannot take any responsibility for the accuracy of the results or for the consequences of how those results are applied.

Ask an expert

To introduce this new purewatercloud service, it is currently being offered at no charge.  You will receive a short answer to a submitted question if it is found to be appropriate for posting due to its particular content.  The posting will not include identifying information.  (See examples below.)

Please contact purewatercloud for a quotation if you would like more extensive consulting services. 

(612) 741-2001

Hours of Operation
Monday - Friday: 8:00 a.m. - 5:00 p.m. (US-CST)

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Water System


Q: Does it make sense to use a tighter cartridge filter upstream of an RO system to reduce the membrane fouling rate?

A: The tighter the filter porosity, the more the filter will cost and the more quickly it will need to be replaced.  Because cartridge filters have a relatively low dirt-holding capacity compared to RO membrane elements, a significant number of filters would need to be replaced before they would have a significant impact on the RO fouling rate.  But if it is simply not feasible to effectively clean an RO onsite due to water demand, system design, or because of the nature of the fouling particles (such as heavy clay), then it may be cost effective to invest in cartridge filters or in improved upstream filtration.

Q: Should I clean my RO first with an acidic solution or an alkaline solution?

A: Cleaning first with an acidic solution will remove metals that tend to act as coagulants in binding together clays and biological materials.  But it is critical that only a moderately low  pH be used, such as 3.5 to 4.0, to prevent stabilization of any biological materials.  Cleaning first with an alkaline solution is a good approach when biological materials might prevent good chemical contact with sulfate scale, which then enables the use of a second step with an aggressively low pH.  An advantage of finishing with a low pH solution is a temporarily tightened membrane that provides better salt rejection at the expense of some of its normalized permeate flow rate.

Q: What is fouling my RO system if I have high SDI values but the spent SDI filter looks clean and my cartridge filters last more than a month?

A: These are common symptoms of the presence of fine biological polysaccharide particles.  They might be generated in an upstream storage tank if not sufficiently chlorinated, or they might originate in a surface water source.  If the natural water is chlorinated prior to filtration, it can break up the polysaccharides into small particles that are even more difficult to filter.  These materials are difficult to clean if allowed to accumulate on an RO membrane.

Q: My RO membrane elements are about three years old.  Should I replace them?

A: Age is not a good basis for replacing RO membrane because they may last ten years or more if well maintained.  But if the salt rejection or the permeate flow rate has declined so that the purified water no longer meets process quality or flow demands, then they should be replaced.  If downstream ion exchange is depleting more frequently, then the question is best answered by the return on investment (ROI).  If permanent fouling results in the need for more frequent cleaning, then the increased cost of the cleaning should be included in the ROI calculation.


We are pleased to work with the following companies:

Technical Books

Tall Oaks Ventures... publishes and distributes technical books for the water treatment industries.

Learning Managemen Systems

MyQuest... Learning Management Systems for presenting your training courses.